1. Field of the Invention
This invention relates to a process for preparing a liquid silicone composition which ensures a low initial viscosity and is able to suppress an increase of viscosity as time passes during the course of storage and which has improved and lasting release properties when cured.
2. Description of the Prior Art
A number of processes for preparing silicone rubber compositions which make use of silica fillers having a specific surface area of not smaller than 50 m.sup.2 /g have been heretofore proposed. For instance, silica fillers are treated with various types of silazane compounds thereby providing silica particles having active surfaces as set out in Japanese Patent Publication No. 58-8703, U.S. Pat. Nos. 4,152,315 and 4,427,801 (corresponding to Japanese Patent Publication No. 63-46786).
On the other hand, for the addition of reactive group-free diorganopolysiloxanes to compositions, a process has been proposed wherein a silicone oil is added to a silicone elastomer composition so as to improve the release properties of the resultant silicone rubber cured product (Japanese Laid-open Patent Application No. 56-106263 (corresponding to British Patent No. 2067471A).
Japanese Patent Publication No. 61-4855 proposes a composition for heat-fixing roll wherein a mixture of aerosil, a hydrophillic silica filler, and a vinyl group-terminated silicone oil are initially thermally treated in the presence of hexamethyldisilazane, to which functionality-free dimethylpolysiloxane is added thereby obtaining the composition. The roll formed of the composition exhibits good releasing properties.
However, the releasing properties of the silicone rubber compositions of the prior art are not satisfactory, coupled with the problem that when the compositions are allowed to stand over a long time (as accelerated under heating conditions), the viscosity inevitably rises.
The fabrication of duplicates using a silicone rubber mold as a reverse matrix has been well known in the art. In practice, such duplicates have been in use as some parts of automobiles and household electric appliances, art objects, and daily necessaries and miscellaneous goods. To this end, a high precision is required for precision parts. From the economical standpoint, importance is placed on the retention of release properties or release durability. On the other hand, a more complicated configuration is duplicated for art objects. This requires improvements of working properties such as a low viscosity and a high tear resistance.
Many attempts have been made on the improvement of the release durability. Fundamentally, reinforcing silica fillers subjected to hydrophobic treatment are used for this purpose. For instance, various types of silazanes are used to treat reinforcing silica therewith. Examples of such silazanes include (CH.sub.3).sub.3 SiNHSi(CH.sub.3).sub.3, (CH.sub.3).sub.3 SiNHSi(CH.sub.3).sub.2 NHSi(CH.sub.3).sub.3, and (CF.sub.3 CH.sub.2 CH.sub.2)(CH.sub.3).sub.2 SiNHSi(CH.sub.2 CH.sub.2 CF.sub.3)(CH.sub.3).sub.2. In order to improve the tear strength, other types of silazanes may be used in combination with the above-mentioned ones, including (CH.sub.3).sub.2 (CH.sub.2 .dbd.CH)SiNHSi(CH.sub.3).sub.2 (CH.dbd.CH.sub.2), (CH.sub.3).sub.3 SiNHSi(CH.sub.3)(CH.dbd.CH.sub.2) OSi(CH.sub.3).sub.2 !.sub.n NH(CH.sub.3).sub.3, and CH.sub.2 .dbd.CHSi NHSi(CH.sub.3).sub.3 !.sub.3. Moreover, hydrophobic silica is also re-treated with these silazanes. It has been found that by these treating procedures, the release durability is improved over those of the cases where non-treated silica fillers are used.
However, the release durability of the existing silicone rubber matrix materials is not satisfactory on comparison with those of epoxy resins and urethane resins which are currently, predominantly employed as a cast resin material. On the other hand, duplicates mainly made of polyester resins are good with respect to the release properties, but with the problem on the working properties such as an initial viscosity of composition.